Doctoral thesis: On the Role of Process Regions at Stationary and Growing Cracks.

Studies of crack behaviour and fracture are generally performed using continuum analysis. Thus, the process region is either not considered at all or assumed to be point-sized. In the latter case it is often ascribed a desired property, usually the ability to consume energy.A major step towards the understanding of the fracture process region the concept of autonomy of the near tip field was introduced by Barenblatt in 1962. He also introduced a cohesive modulus as a description of the material behaviour in the crack tip vicinity. This implies again that process region should be small in some sense. In an elastic surrounding smallness of the process region must be related to a significant geometrical length, usually the crack length. Which introduces the question about what happens in the case the crack is of similar size or even small compared with the linear extent of the fracture process region. The present thesis brings some light into this

Paris' exponent m<2 and behaviour of short cracks - Discussion of fracture paper #23

The official iMechanica Blog for the Elsevier Journal Engineering Fracture Mechanic

Crack paths and fracture process region autonomy - Discussion of fracture paper #12

The official iMechanica Blog for the Elsevier Journal Engineering Fracture Mechanic

A One-Step Solution Technique for Elastic-Plastic Self-Similar Problems

At self-similarity the strain history is implicit in and deducible from the strain field at each instance of the loading process. This fact is taken advantage of in a FEM-technique which allows the load to be applied in one single step, only, even when the incremental theory of plastic flow has to be used.Two self-similar problems are solved. Firstly an analytically solvable anti-plane strain crack problem is treated and the numerical one step solution is found to agree very well with the analytical one and significantly better than a step-by-step solution. Secondly a mode I crack problem for asymptotic small scale yielding in plane stress is examined. The result suggests that the plasticity correction for the crack length is significantly less than estimated by Tada et al. [1]

A fracture mechanics model to study indentation cutting

Many cutting processes, such as chopping, slicing, and carving, consist in 2 different stages: an initial stage of indentation, in which the cutting tool is pushed into the material under the action of an external force, and a second stage, where the target material undergoes a progressive separation. This second stage is characterised by the formation of a fracture surface followed by the cut propagation due to the increasing external force, until eventually a steady state might occur. The purpose of this paper is to analyse the cutting process by means of some concepts of fracture mechanics and discuss the occurrence of the steady state. A simple model is used to obtain an analytic expression of the stress intensity factor at the tip of the cut and investigate the evolution of the fracture process. It is found that the cut propagation depends on the wedge sharpness. The analytic results are compared with finite element analyses, where the effect of tip blunting due to plasticity is taken into account. The influence of the cutting tool geometry is also discussed

on the fracture processes of cutting

Abstract: The process of cutting is treated as a fracture mechanical process. For an elliptic rigid wedge pressed into an elastic material, fracture may occur as an autonomous process if the tip of the wedge is sufficiently blunt or is affected by the geometry of the wedge if the tip is sharp. The conditions leading to the former or the latter case is obtained as a relation between the wedge tip radius, the fracture toughness and the modulus of elasticity. These limits and the intermediate states are discussed. The implications of the drastic changes of the mechanical state of the near tip region when the wedge edge is sharp are also discussed

On dissolution driven crack growth

AbstractThe formation and growth of a crack in a body subjected to stress driven material dissolution is studied. The rate of material dissolution is proportional to strain energy and curvature of the body surface. The formation of a crack from a plane surface is preceded by an evolving surface roughness. The continued dissolution enhances roughness amplitude resulting in pit formation. As the pit grows deeper into the material, it assumes the shape of a crack. The sharpness of the crack reaches its maximum during this transition from a pit to a crack. As the crack grows, a self-similar state is gradually assumed. During this phase characteristic lengths of the crack shape scale with the crack length. In line with this the crack progressively becomes blunt. The widest part of the crack when unloaded is in the vicinity of the crack tip. A consequence of the model is that no criterion is needed for crack growth. Neither is a criterion needed for determination of the crack path. It also follows that the crack growth rate is almost independent of the remote load. Further, spontaneous crack branching is anticipated. A motivation for this is given